The present invention relates to a winding conductor for dynamoelectric machines, and more particularly to a transposed, stranded conductor of high current capacity for use as a winding conductor or half-coil for machines of large size, such as turbine generators.
The winding conductors of large dynamoelectric machines, such as turbine generators, are placed in slots in a laminated magnetic stator core. Each conductor, or half-coil, has a straight portion disposed in the slot and end portions which extend around the circumference of the core for connection to other conductors lying in different slots to form complete coils. The magnetic fluxes which occur across the slots cause induced voltages and eddy currents in the conductors, and for this reason the conductors of large machines are always of stranded construction, being built up of a substantial number of relatively thin strands to minimize the eddy current loss. Since the fluxes vary radially, the induced strand voltages vary throughout the slot so that circulating currents can occur because of the unbalanced voltages. For this reason, it is necessary to transpose the strands to cancel out as far as possible the unbalanced strand voltages to minimize circulating currents and the resultant heating.
The most usual type of transposition which has been used for many years is the Roebel transposition. In this arrangement, as shown in Roebel U.S. Pat. No. 1,144,252, the strands are disposed in two side-by-side stacks and transposed within the slot by crossovers between the stacks. That is, the strands are inclined in each stack so that each strand moves radially to the top or bottom of the stack, crosses over to the other stack, moves radially through the stack, and crosses over back to the first stack. Thus, looking at the end of the conductor, each strand moves through an angle of 360.degree. in going from one end of the slot to the other, and emerges in the same relative position at which it entered the slot. Each strand, therefore, occupies all positions in the slot for equal distances and the induced strand voltages are exactly balanced within the slot. Induced strand voltages also occur in the strands in the end portions of the conductor outside the slot, however, and in the usual practice these have been taken care of by the group transpositions between groups of strands at the connections between the ends of adjacent conductors, so that these voltages are balanced out within a complete coil or group of coils.
The Roebel transposition with group transpositions between conductors is very satisfactory where the strands are insulated from each other throughout each coil or group of coils. In many cases, however, especially in machines of large size, it is necessary or desirable to solidly join the strands together at each end of a single conductor or half-coil, as in liquid-cooled machines where water or other liquid coolant must be circulated through each conductor from one end to the other, or in cases where the currents to be carried are large enough to require more than two stacks of strands. In such cases, where group transpositions cannot be made, the induced strand voltages in the end portions must be taken care of in other ways. One way of doing this is by means of a 540.degree. transposition within the slot, as shown in Ringland U.S. Pat. No. 2,281,641, which results in inverting the strands in the two end portions with respect to each other to effect at least partial cancellation of the end portion strand voltages. A more effective construction which substantially completely eliminates circulating currents due to unbalanced strand voltages is shown in Brenner U.S. Pat. Re. No. 27,489. In this arrangement, the strands are transposed through 540.degree. in the slot and voids or untransposed sections of suitable length are interposed at proper points in the conductor so that unbalanced strand voltages occur within the slot which balance out the strand voltages in the end portion.
As mentioned above, in many cases it is necessary to provide four stacks of strands to carry the high load currents involved in modern machines of large size. The conventional way of doing this is to place two standard Roebel bars in parallel in each slot, and if the strands are insulated from each other throughout the complete coil, this is entirely satisfactory. In cases such as those mentioned above, however, where the strands are shorted together at each end of each conductor, this arrangement is not satisfactory. It might appear that if four stacks of strands are needed, two 540.degree. bars as discussed above could be placed side-by-side and joined together at both ends. In actual practice, however, it is found that the radial fluxes in the machine are such that voltages are induced, especially in the end portions of the conductors, which cause circulating currents to flow between the two paralleled bars which are superimposed on the normal load current. These circulating currents are large enough to cause seriously unequal distribution of the total current, so that one of the parallel bars carries a considerably larger current than the other, and can seriously overheat because of the greatly increased losses as compared to the other bar.
One way of dealing with this situation is, of course, to increase the cooling of at least the bar with the greater current, and this has been done by increasing the pressure of the coolant water which circulates through the bar to increase the flow and thus remove the additional heat. This is not a satisfactory solution, however, since the maximum permissible pressure of the water must be less than the hydrogen pressure in the machine to prevent leakage of water into the machine in case of a leak in the coolant system. It has also been proposed to overcome this problem by transposition schemes, such as that of Pannen U.S. Pat. No. 3,280,244 where two central stacks of strands are transposed in the usual manner and two outside stacks of strands are transposed around the first two in a concentric arrangement, or by the four stack transposition of Heller et al U.S. Pat. No. 3,647,932 in which the strands are transposed through all four stacks of strands. These arrangements, however are difficult and expensive to manufacture and are undesirable for that reason.